The present invention relates to a vertical cavity surface emitting laser and more particularly, the present invention relates to a top emitting laser with top electrode current injection having an improved optical mode profile and power handling.
Generally speaking, the known laser designs used so far have injected the current by means of a ring contact. However, the diameter of the device is normally larger than the vertical distance from the electrode to the active, light-emitting area. This limits the current spreading and since lasing occurs preferentially around the circumference of the device, several limitations are realized including:
i) design complications in high power lasers by increasing the area, since the maximum output power is proportional to the circumference and not to the area;
ii) coupling difficulties with the output of the light into the optical fibre since the fibre is emitted from a ring pattern along the circumference;
III) low efficiency since lasing takes place close to the current blocking region which surrounds the active volume. This area, whether oxidized, implanted, or designed in other ways, has a higher degree of defects; this leads to low quantum efficiency;
iv) high resistance of the device;
v) a mode pattern which is uncontrolled and varies with the forward current and between individual devices;
vi) high variation of output power between different devices; and
vii) a process which requires high precision when defining the electrode pattern.
In the art, current designs position the ring contact outside the emitting area and include a current blocking layer to force the current into the active volume. The current blocking layer may be ion implantation as taught in, for example, U.S. Pat. No. 5,115,442, issued May 19, 1992 to Lee et al. or by oxidation as set forth in U.S. Pat. No. 5,262,360, issued Nov. 16, 1993, to Holonyak, Jr. et al. More frequently employed is a ring electrode partly covering the active, light-emitting area. These designs may be combined with a layer(s) of graded resistivity with varying high and low resistivity to improve the current spreading. Current spreading is described in U.S. Pat. No. 5,343,487, issued Aug. 30, 1994, to Scott et al.
In view of the limitations of existing methodology, there exists a need for greater efficiency in top emitting laser power output. The present invention covers this need and provides a solution as well as a way to control the optical mode profile.
One object of the present invention is to provide an improved VCSEL with enhanced current spreading.
In accordance with a further object of the present invention, there is provided a light-emitting device, comprising:
a substrate;
a first layer of dielectric mirrors;
an active layer;
a passivated region;
a second layer of dielectric mirrors;
a bonding pad; and
an electrode connected to the pad and in electrical contact with the second layer of dielectric mirrors.
The top electrode of a top emitting VCSEL may take any shape which facilitates current injection in the active region. In the example, a circular electrode with fingers which overlie the conducting, active area is provided, however, it will be appreciated by those skilled that variations achieving the same result, may be used. In terms of variations, the electrode may comprise an arrangement with any number or shape of the fingers toward the center of the device to improve current spreading. The electrode may take a pattern which improves the mode pattern in the device. Further, the electrode may be unsymmetrically, for example, striped to improve the polarization of the device.
Other variations in the design of the device include positioning an inner ring coaxially with the electrode connected to the bond pad. It has also been found that the inventive concept set forth herein may be applied to non-circular forms for the emitting area.
The design of the top contact can be made to enhance polarization to get a higher degree of polarization on the emitted light. The design can be used for any electro optical device, i.e. micro cavity light-emitting diodes (MCLED""s or RECLED""s). Regarding fabrication, the contacts are normally defined by photolithography, on the top surface; designs using other production methods like electron beam lithography as well as the other suitable techniques may be employed.
A further object of the present invention is to provide a method of enhancing the output power of a light-emitting semiconductor, comprising:
providing a top emitting semiconductor laser having Bragg mirrors, an active region, a passivated region, substrate and bond pad connected to an electrode whereby applied current is concentrated in the active region for increased output power of the semiconductor.
Having thus generally described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.